Scrapie is a spongiform encephalopathy of sheep and goats which can be transmitted experimentally to several other animal species. Similar diseases are recognized in cattle and humans. No etiologic agent has been identified. However, the proteinase K resistant form (PrP-res) of an endogenous protein designated prion protein (PrP) purifies with infectivity and is important to disease pathogenesis. We developed a sensitive assay for PrP-res and utilized it to diagnose scrapie in sheep. Analysis based on PrP-res detection was much more accurate and less subjective than the currently used method of diagnosis based on the microscopic evaluation of brain. We also showed that PrP-res analysis of spleen or lymph node was nearly as accurate as analysis of brain. We are determining if PrP-res accumulates prior to clinical disease in lymph node. If so, an ante mortem test for infection may for the first time be possible. We are also using PrP-res analyses to test tissues from cattle in order to determine if spongiform encephalopathy currently exists in U.S. cattle and, thereby, whether an epidemic similar to BSE in Great Britain is possible in the U.S.A. PrP-res analysis should also be relevant for diagnosis of the human disease counterparts. We are also using a scrapie-susceptible mouse neuroblastoma cell line (MNB) to study the influence which specific PrP gene sequences have on interspecies transmission of spongiform encephalopathies. To do so, we have expressed various mouse-hamster PrP constructs in the scrapie- infected cells and are now analyzing them in mice and hamsters to determine if species tropism has been altered. Similar studies could tell us if BSE represents a risk to humans. In addition, the MNB cells as well as a pluripotent embryonal carcinoma cell line (P-19) are being used to study the normal function of the PrP protein as well as factors which might account for the biochemical changes which lead to the conversion of the endogenous PrP protein to the PrP-res form.